There are innumerable instances where mixing of materials enclosed within some form of container is required. Such materials are, of course, miscible and may be in the form of gases, liquids, granular, particulate, or some other form of fluent materials. As one example with which the present invention is expecially concerned, in the production of mushroom spawn, a small quantity of mycelium is introduced in a much larger quantity of grain and enclosed in a flexible bag where growth can occur. However, to establish a thorough distribution, mixing of the materials within the bag is requisite. Most commonly, such mixing is accomplished by manual manipulation of the flexible bag which, as will be obvious, is a tedious operation that is not only inefficient, but also is not too effective.